DC Generators
Application criteria for DC generators are discussed next.
Service Conditions
The DC generators should be correctly selected with respect to their service conditions.Theseconditionsmaybeusualorunusual,involvingenvironmental and operating conditions. The typical service conditions that have been listed under Section 10.4 also apply to DC generators.
Ratings
The DC generators are classified into industrial DC and other DC integral- horsepower generators and large-apparatus DC generators (larger than 0.6 kW/rpm) of open type. The industrial generator ratings range from 0.75 to 720 kW, speeds range from 720 to 3450 rpm, and voltages range from 125 to 500 V. The large-apparatus DC generator ratings range from 125 to 6400 kW, speeds range from 200 to 900 rpm, and voltages range from 250 to 700 V. Refer to NEMA standards MG1, Section II, Part 15 and MG1, Section III, Part 24 for specific information on generator size, speed, and voltage ratings.
Temperature Rise
The temperature rise of DC generators under rated load conditions is dependent on the type of insulation system and enclosure used for the various parts of the machine. Either the thermometer or the resistance method is used for the measurement of temperature rise. Refer to NEMA standards MG1, Section II, Part 15 and MG1, Section III, Part 24 for specific information on temperature rise for the two types of generators.
Overload Capability
Industrial-type generators are capable of carrying overload for 1 min, with successful commutation loads of 150% of the continuous rated load amperes at rated load excitation. Large-apparatus DC generators of open type are capable of carrying 115% of rated current for 2 h and 200% of rated current for 1 min at rated speed and rated or less than rated voltage.
Voltage Excitation
Large-apparatus DC generators, when operated at less than rated voltage, shall carry currents equal to those corresponding to their kilowatt and voltage ratings. The load voltage at rated load of a self-excited, flat, compound-wound, drip-proof industrial generator, rated at 50 kW and smaller and employing class B insulation, shall not exceed 112% of the rated voltage at rated load.
Overspeed
DC generators are constructed so that, in an emergency, they will withstand overspeed of 25% without mechanical injury.